I would not add the eggs when she's newly broody, personally. She may not know when to stop turning them & go into "lock down", despite their peeping. The best possible scenario is to have the hen's cycle of brooding match the eggs' cycle of development. That is, the same number of days along in broodiness as the eggs are developing. If you do incubate them to hatching and put the just-hatched chicks under her, she may reject them because she hasn't sat for 21 days. That's the safest thing to try, though.
I've recently had great success with broody adoption of chicks by putting the broody in a quiet (indoor) nest box at the time the eggs go in to lockdown and using a heated pet pad under her nest box, a cat carrier.
I've gotten hens who have sat about 16 days to feel like adopting the chicks this way. I think the heat on their belly gives their hormones a broodiness-boost.

I almost always add incubated eggs to my broodies. I hate to see them sit on a nest for 3 weeks and lose weight. Once I placed eggs under one of my girls that were only about 4 days away from hatching. She was a great little mother to them. I guess she just figured they were fast growing eggs!

I have tried placing day-old chicks under a broody before - she attacked them and killed one!

I wondered about this myself....I had a broody this year, and gave her some eggs, and she did really well for a couple of weeks, then I discovered she would get confused about which nest she was setting on, and she would go sit on some eggs in the box next to hers that the hens had just layed.

Her eggs never hatched.......I tried to lock her up in another area, but she refused to set when I did that........

I've tried it both ways, given partly-incubated eggs (2 week mark I think) to a hen who had been brooding on infertile eggs... given newly-hatched chicks to another such hen... and have even successfully given shipped day-old chicks to a determined broody hen several times. I wasn't sure about that last one but it has worked out well. They seem to welcome little fluffies under them, but don't like to see them. There was maybe a 4 - 5 hour period where any chick that showed its face in the open got a "yarr" sound and peck in the butt. After 6-8 hours shut in a dark box with snuggly chicks underneath her (I did it in shifts so the chicks all got a chance to eat and drink), the broody's "yarr" had changed to "churrrr, churrr" and she was actively trying to show them chicken food.

I agree that it's nice for the hen to cut short the incubation period. The ones I've done this with don't seem to question their good luck, when their eggs miraculously hatch after 7 days. Or 4. It's very important, though, that the hen has well and truly quit laying. Early on, a broody will still be adding eggs to the nest (at least, mine do). If she still feels like laying eggs, she won't want to leave the nest and may ignore the chicks entirely.

Chickens cannot count very well. They run out of fingers and toes too quickly. They do not stop being broody after 21 days and they sure do not seem to notice if the number of eggs under them change.

Personally I would give her a few of the eggs. I don't know how many she can cover, but try to give her some that are developing with the thought that some will hatch under her. Then, depending on her size and how many chicks actually hatch, you can look at giving her more or maybe all of them to raise. As long as she can cover them, she should be able to raise them. I had a Black Australorp raise 15 chicks doing something real similar to this last year.

I think the odds of her accepting the chicks is higher if some hatch under her. They will talk to each other while the chicks are still in the shell. A good broody will accept about any very young chick you slip under her, especially if you do it at night. But not all broodies are good. Sometimes bad things happen. You are dealing with a living animal so anything can happen.

I just had 3 broodies (1 EE & 2 Ameraucanas) co-hatch chicks in one nest. They started with 10 eggs and kicked out 5 clears after a week. Somehow 2 more disappeared leaving them with only 3 eggs. I had eggs in the bator that were due to hatch about the same time so I gave them 6 more when I locked down the bator. They hatched out all 9 eggs they had ~ but only 3 of 5 left in the bator hatched ~ so when the chicks were 1 day old I took the 3 out and tucked them under the Mommas and now they are the proud Mommas of 12 blue and Splash Ameraucanas. They are all just 1 year old and first time broodies so I waited until the end to give them the extra eggs and babies to make sure they were going to stick with it.